Study On Stability Of HLW Repository In Tamusu Clay Effected By Thermo-Mechanical Coupling

During the operation of a deep geological high-level radioactive waste repository,the long-term decay heat release of waste tanks and the coupling of high ground stress in deep formations are issues that cannot be ignored for the safe operation and long-term stability of the repository.Clay rock is considered to be a good candidate wall rock for the repository.France,Belgium,Switzerland,and other countries choose clay rock as the main host rock for the repository.Not until the 21st century,the selection of clay rock sites has received attention.In this paper,a study of the stability of the repository under the coupling of temperature and stress of the Tamusu clay was completed.First,the thermal conductivity of the Tamusu clay was obtained by the heat conduction test,and the influence of temperature on the mechanical properties of Tamusu clay by thermal expansion monitoring,uniaxial compression at high temperature,and triaxial compression at high temperature;secondly,the yield surface morphology of Tamusu clay influenced by temperature is analyzed according to the stress space criterion;finally,the thermo-mechanical coupling of the repository was carried out by numerical simulation,to evaluate the stability of the repository.The main research results are as follows:（1）Thermal conductivity test was carried out for the clay rock in the upper section of Bayingebi Formation in Tamusu,and the test results showed that:The variation range of the thermal conductivity of the Tamusu clay rock is between 0.869 W/（m·K）～1.751W/（m·K）,and the overall trend is increasing with the depth,which can be expressed by y=0.0009x+0.7534.Thermal conductivity of clay rock changes due to the difference in structure and mineral composition of clay rock;（2）Conduct thermal conductivity tests on clay rocks near the target depth,and the test results showed that:The thermal conductivity of the rock sample near the depth of 542m is an average thermal conductivity of 1.120 W/（m·K）,an average specific heat capacity of778.407 J/（kg·K）,and an average thermal diffusion coefficient of 0.629 mm²/s;The average thermal conductivity coefficient of the rock sample near the depth of 573m is1.638 W/（m·K）,the average specific heat capacity is 690.98 J/（kg·K）,and the average thermal diffusion coefficient is 0.959 mm²/s;Compared with foreign clay rocks,the thermal conductivity of Tamusu clay rocks is similar to that of Callovo-Oxfordian clay rocks in France and Opalinus clay rocks in Switzerland,but weaker than that of Boom clay rocks in Belgium;（3）Conduct thermal expansion monitoring,high-temperature uniaxial and triaxial compression tests according to the influence of temperature on the mechanical properties of the Tamusu clay rock.The test results show that:The axial linear thermal expansion coefficient of Tamusu clay is 2.40～5.03×10^-5℃^-1;the radial linear thermal expansion coefficient is 0.82～3.09×10^-5℃^-1;The confining pressure increases the strength and elastic modulus of the Tamusu clay rock,and the deformation ability is significantly improved,and the failure property gradually changes from uniaxial brittle failure to ductile failure;the Tamusu clay rock,the failure mode is mainly single-shear failure,and splitting failure may occur in the uniaxial case.After the confining pressure is applied,a tensile or shear crack will be generated in the rock block after the rock sample is damaged.The angle between the extension direction of the crack and the axis of the rock sample increases with the increase of the confining pressure;The changes in mechanical strength such as the elastic modulus and compressive strength of Tamusu with increasing temperature are mostly broken lines with inflection points,and the inflection points mostly appear at 75°C.（4）The high-temperature triaxial compression test of Tamusu shows that when the confining pressure is constant,the effect of temperature increase on the spatial yield state of Tamusu clay is reflected in the change of the angleβbetween the stress vector and the principal stress axis;Theβangle increases with the increase of temperature.Combine the expression ofβangle with the Mohr-Coulomb yield criterion to establish a mutual conversion relationship;According to the relationship,the spatial yield surface of Tamusu clay rocks at different temperatures is complemented and found that as the temperature increases,the yield surface gradually"shrinks".（5）Based on the thermal-mechanical coupling parameters of Tamusin clay rock and the design model of the Belgian clay surrounding rock repository,the heat release rate of the waste tank was corrected based on the research on the heat release rate of high-level radioactive waste in Sweden and Japan,and the results showed the temperature and maximum principal stress on the surrounding rock wall reached the maximum when the waste tank was buried at 6.5a,and the deformation of the surrounding rock reached the maximum at the 10a;During the simulation process,there was no damage to the repository and the waste tank,and there was no problem of deformation and instability of the surrounding rock.During the simulation process,there was no damage to the repository and the waste tank,and there was no problem of deformation and instability of the surrounding rock.However,the temperature of the surrounding rock wall reached 125°C,which exceeded the safe design temperature.Therefore,the repository model is not suitable for Tamusu clay rock,and the design of the repository still needs to be optimized.